It is generally known that antenna performance is dependent on the antenna size, shape and the material composition of certain antenna elements, as well as the relationship between the wavelength of the received/transmitted signal and certain antenna physical parameters (that is, length for a linear antenna and diameter for a loop antenna). These relationships and physical parameters determine several performance characteristics, including: input impedance, gain, directivity, polarization and radiation pattern. Generally, for an operable antenna, the minimum effective electrical length (which according to certain antenna structures, for example antennas incorporating slow wave structures, may not be equivalent to the antenna physical length) must be on the order of a quarter wavelength or a multiple thereof of the operating frequency. A quarter-wave antenna limits the energy dissipated in resistive losses and maximizes the energy transmitted. Quarter and half wavelength antennas are the most commonly used.
The radiation pattern of the half-wavelength dipole antenna is the familiar omnidirectional donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical antenna gain is about 2.15 dBi.
The quarter-wavelength monopole antenna placed above a ground plane is derived from a half-wavelength dipole. The physical antenna length is a quarter-wavelength, but when placed above a ground plane the antenna performance resembles that of a half-wavelength dipole. Thus, the radiation pattern for a quarter-wavelength monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
Printed or microstrip antennas are constructed using the principles of printed circuit board techniques, where one or more of the metallization layers or interconnecting vias serve as the radiating element(s). These antennas are popular because of their low profile, ease of manufacture and low fabrication cost. One such antenna is the patch antenna, comprising a ground plane below a dielectric substrate, with the radiating element overlying the substrate top surface. The patch antenna provides directional hemispherical coverage with a gain of approximately 3 dBi.
The burgeoning growth of wireless communications devices and systems has created a need for physically smaller, less obtrusive and more efficient antennas that are capable of wide bandwidth and/or multiple frequency operation. As the size of physical enclosures for pagers, cellular telephones and wireless Internet access devices shrink, manufacturers continue to demand improved performance, multiple operational modes and smaller sizes for today's antennas.
Smaller packaging envelopes do not provide sufficient space for the conventional quarter and half wavelength antenna elements. Also, as is known to those skilled in the art, there is a direct relationship between antenna gain and antenna physical size. Increased gain requires a physically larger antenna, while users continue to demand physically smaller antennas with increased gain.
With the expansive deployment of computer resources, it has become advantageous to connect computers to allow collaborative sharing of information. Conventionally, the connection is in the form of wired computer or data networks (generally referred to as local area networks or LAN's) operating under various standard protocols, such as the Ethernet protocol. Users connected to the network can exchange data with other network users, irrespective of the physical distance between, the users. These networks, which have become ubiquitous among computer users, operate at fairly high speeds, up to about 1 Gbps, using relatively inexpensive hardware. However, LANs are limited to the physical, hard-wired infrastructure of the structure in which the users are located.
During recent years, the market for wireless communications of all types has enjoyed tremendous growth. Wireless technology allows people to exchange information using pagers, cellular telephones, and other wireless communication products. With the steady expansion of wireless communications, wireless concepts are now being applied to data networks, relieving the user of the need for a wired connection between the computer and the network.
The major motivation and benefit from wireless LANs is the user's increased mobility. Untethered from conventional network connections, network users can access the LAN from wireless network access points strategically located within a structure or on a campus. Depending on the antenna gain, available signal power, noise and interference, wireless local area networks can operate over a range of several hundred feet to a few thousand feet. Frequently it is more economical to install a wireless LAN than to install a wired network in an existing structure. Wireless LANs offer the connectivity and the convenience of wired LANs without the need for expensive wiring or rewiring.
The Institute for Electrical and Electronics Engineers (IEEE) standard for wireless LANs (IEEE 802.11) sets forth two different wireless network configurations: ad-hoc and infrastructure. In the ad-hoc network, computers are brought together to form a network “on the fly.” There is no structure to the network and there are no fixed network points. Typically, every node is able to communicate with every other node. The infrastructure wireless network uses fixed wireless network access points with which mobile nodes can communicate. These wireless network access points are typically bridged to landlines to allow users to access other networks and sites not on the wireless network.
The IEEE 802.11 standard governs both the physical (PHY) and medium access control (MAC) layers of the network. The PHY layer, which handles the transmission of data between nodes, can use either direct sequence spread spectrum, frequency-hopping spread spectrum, or infrared (IR) pulse position modulation. IEEE 802.11 makes provisions for data rates of either 1 Mbps or 2 Mbps, and calls for operation in the 2.4-2.4835 GHz frequency band (which is an unlicensed band for industrial, scientific, and medical (ISM) applications) and 300-428,000 GHz for IR transmission.
The MAC layer comprises a set of protocols that maintain order among the users accessing the network. The 802.11 standard specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this protocol, when a node receives a packet for transmission over the network, it first listens to ensure no other node is transmitting. If the channel is clear, the node transmits the packet. Otherwise, the node chooses a random “backoff factor” that determines the amount of time the node must wait until it is allowed to retry the transmission.
Several extensions of the IEEE 802.11 standard have been developed. The first, referred to as 802.11a, provides a data rate of up to 54 Mbps in the 5 GHz frequency band. The 802.11a standard requires an orthogonal frequency division multiplexing encoding scheme, rather than the frequency hopping and direct sequence spread schemes of 802.11. The 802.11b standard (also referred to as 802.11 high rate or Wi-Fi) provides a 11 Mbps transmission data rate, with a fallback to data rates of 5.5, 2 and 1 Mbps. The 802.11b scheme uses the 2.4 GHz frequency band, using direct sequence spread spectrum signaling. Thus 802.11b provides wireless functionality comparable to the Ethernet protocol. The newest standard, 802.11g provides for a data rate of 20+ Mbps in the 2.4 GHz band. A primarily European wireless networking standard similar to the 802.11 standards, referred to as HyperLAN2, operates at 5.8 MHz.
Today, devices implementing either the 802.11a or 802.11b standard are available. The higher data rate of 802.11a devices can support bandwidth hungry applications, but the higher operating frequency limits the radio range of the transmitting and receiving units. Typically, 802.11a compliant radios can deliver 54 Mbps at distances of about 60 feet, which is far less than the 300 feet radio range over which the 802.11b systems can operate, albeit at lower data rates. Thus 802.11a installations require a larger number of media access points from which users link into the network.
Recognizing the transient nature of a wireless signal link due to movement of the communicating devices relative to each other (typically, the base station antenna is permanently mounted while the portable device with its attendant antenna is movable relative to the base station antenna), and the time varying properties of noise that can affect system performance, various schemes have been proposed to ensure that signals are received over the link with a sufficient ratio of bit energy to noise spectral density to allow recovery of the data. Antenna spatial diversity is one such scheme, employing two antennas at the transmitting and/or receiving device, with selection of the operative antenna based on one or more monitored signal quality metrics. Thus, for example, the antenna providing the largest signal power or signal-to-noise ratio can be selected as the operative antenna. The primary objective of an antenna diversity system is to reduce signal fading caused by multipath signals that can coherently cancel at the antenna, thereby reducing the received signal quality and making signal decoding more difficult and prone to error. For example, as a portable unit employing a single antenna is moved or as the signal path changes dynamically in length and/or angle due to motion of the scattering or reflecting surfaces relative to the portable unit, the multipath signals received at the antenna can destructively interfere. (The signals can also constructively interfere.) In addition, the transmission medium itself (the atmosphere) can produce variations that are manifest as fades at a receiver employing only a single antenna.
In the prior art spatial diversity system the maximum allowable distance between the antennas is dependent on the available space. For example, if the antennas and associated receiving and transmitting circuitry are assembled onto a PCMCIA card for insertion into a laptop computer, then the separation will be on the order of a few inches. If the antennas are mounted for use with a desktop computer the spatial separation can be on the order of several inches or a few feet. Although these dimensions can be on the order of a fraction of a wavelength at current wireless frequencies, the use of spatially diverse antennas can still achieve improved performance.
The signals received at two spatially diverse antennas differ in phase and amplitude due to the distance between the antennas. The two received signals can be summed to produce a stronger received signal, or a selection process can determine, based on one or more predetermined received signal metrics, which of the two antenna signals should provide the input to the receiver circuitry (or which of the two antennas should transmit the signal). Monopole antennas above a ground plane or dipole antennas are conventionally used in these spatial antenna diversity applications.
If a multipoint reception system is used (often called a multi-branch reception system in the art), and the signals are uncorrelated at each branch (for instance, by using separate diverse locations for the antenna reception points as discussed above) the signal fading problem can be reduced. This fade reduction results from the statistical independence of the signal branches, so that as one branch fades, the probability that the other branch is also fading is small.
Polarization diversity is achieved using two linearly polarized antennas mounted orthogonally. Thus the diversity scheme relies upon the independent polarization of two or more reception branches to achieve a reduction in signal fading. The statistical independence of the branches is due to the changes in electromagnetic wave polarization as the waves are scattered and reflected along different propagation paths to the receiving antenna.